A typical form of fuel tank, for example for a motor vehicle, comprises a tank body and a fuel conveyor or delivery unit which is arranged within the tank body. The delivery unit has an electric fuel pump by which fuel in the tank can be supplied to the engine of the motor vehicle. In such an arrangement the reservoir whose volume is substantially smaller than that of the fuel tank as a whole is to ensure in particular the function that, even under adverse operating conditions, for example when the motor vehicle in which the tank is fitted is negotiating a bend of considerable length or when the vehicle is not in a horizontal position, there is always a sufficient amount of fuel still in the region of the intake of the fuel pump so that the pump can still draw enough fuel and thus prevent the engine from suffering from fuel starvation. That presupposes the existence of a certain minimum amount of fuel in the reservoir and thus also presupposes that the reservoir itself is of a corresponding minimum volume.
In many cases, the tanks used comprise a thermoplastic material and they can preferably be formed in one piece by an extrusion blow molding procedure. In such a situation the delivery unit with the reservoir is frequently fitted into the tank body after production of the tank body, through an opening which is generally subsequently formed in the wall of the tank after manufacture thereof. As, for reasons of strength and stability and also sealing integrity in respect of the opening which is to be closed after the delivery unit has been fitted in the tank body, it is desirable for that opening to be kept as small as possible, a typical tank of such a configuration suffers from the disadvantage that the reservoir of the fuel delivery unit which is introduced into the tank body through the opening is of a correspondingly small volume which for example is of the order of magnitude of between 0.5 and 1 liter. In many cases, having regard to the duration of the above-mentioned adverse operating conditions which may occur in a practical context and during which only little or no fuel flows into the reservoir from the tank volume surrounding the reservoir, the above-mentioned small volume of the reservoir of the delivery unit is excessively small and thus inadequate for its intended purpose.
It will be noted here that under some circumstances the above-mentioned volume of the reservoir, which is small in any case for the reasons indicated above, may also be further reduced by virtue of the provision of a filling level measuring device which is mounted to the delivery unit on the outside thereof, for measuring the level of fuel in the main body of the tank which surrounds the reservoir. The filling level measuring device thus has to be introduced into the tank through the opening in the tank wall, jointly with the delivery unit. As the cross-section of the opening determines the available contour of the delivery unit and the measuring device may therefore not project outwardly substantially beyond that contour, it thus becomes necessary for the measuring device to be disposed within the contour of the delivery unit, that is to say, within the cross-sectional configuration thereof. That has the inevitable consequence that the volume of the reservoir is correspondingly reduced. Accordingly, the volume of the reservoir is still further smaller than the magnitude which is in any case predetermined by the cross-section of the opening in the wall of the tank body.